The invention regards position sensors in stick form according to a non-contacting functional principle for application in fluids, among other things. Position sensors in stick form measure the position of a position indicator fastened to a component moveable relative to the stick position sensor.
Such sensors are used, among other things, in an interior of hydraulic or pneumatic cylinders in order to know the exact extension of the piston/cylinder unit, which is of great importance for the control of the machinery and equipment operated therewith.
The position sensor is thereby located in a tight housing which comprises a long, slender sensor-stick-housing and a connected, shorter sensor sensor-head-housing which has a larger diameter and wherein the processor electronics are located.
Thereby, the sensor with its sensor-head-housing is located in the piston/cylinder unit in a longitudinal fixed manner so that the slender sensor-stick-housing extends into a typically central bore of the piston or the piston rod where the position indicator is mounted.
Since the sensor-stick-housing, and thereby the measuring length of the sensor, extends along an entire possible extension length of the piston rod, the current position of the piston rod relative to the cylinder is known at any time.
Thereby, the housing of the sensor at its exterior side is in direct contact with operating fluid of the piston/cylinder unit and is also exposed to its operating pressure. Especially in hydraulic units with very high pressures, therefore, it is of great importance that the sensor housing is provided stable and tight and that a sufficient sealing between the housing of the sensor and the piston/cylinder unit is provided, mostly through a respective seal at an exterior circumference of the sensor-head-housing relative to the surrounding wall of the pneumatic or hydraulic cylinder.
As a non-contacting sensor principle thereby differential transformatoric measuring procedures (LVDT's), non-contacting inductive measuring procedures (LVP's), inductive potentiometric measuring procedures (DC/DC-sensors), Eddy current procedures and often also magnetic, in particular magnetostrictive functional principles are being used. With the latter ones a permanent magnet is being used as a position indicator with other processes a tube sleeve, a submerged anchor, or a similar component. With PCLD-sensors through a magnet a virtual air gap is being created in a ferromagnetic core.
As it is well known, magnetostrictive position sensors function as follows:
A wave conductor typically consists of a tube, a wire, or a band and can also serve as an electric conductor. The wave conductor can also be located in a shape generating linear or circular body made from non-magnetic material, such as plastic or metal, for receiving the wave conductor and holding it in bearings.
Based on the Wiedemann-Effect an electric impulse fed into the wave conductor generates a mechanic elastic wave when superimposed with a position magnet.
At a certain location, typically at one end of the wave conductor, in particular, the torsion component of this mechanic/elastic impulse is detected by a detector unit, mostly located in a fixed position relative to the wave conductor. The duration between the triggering of the electrical excitation impulse and the reception of this mechanic-elastic wave, thereby, is a measure of the distance of the slideable position element, e.g. of a position magnet, from the detection device.
A typical such sensor is described in the U.S. Pat. Nos. 5,590,091 and 5,736,855.
Subsequently, only magnetostrictive position sensors are referred to without limiting the invention to this position measurement principle.
In such position sensors used in piston/cylinder units there are several problem areas.
One problem area is an increase of overall length of the piston/cylinder unit through the position sensor.
While the slender sensor-stick-housing extends into an interior of the piston rod, the wider sensor-head-housing requires a respective interior length in the cylinder of the piston/cylinder unit for housing, which increases the overall length of the piston/cylinder unit.
It is attempted to also shorten the sensor-head-housing in axial direction through miniaturization of the processing electronics housed in the sensor-head-housing. However, with most position sensors only a single cable exit direction is offered for the position signals from the sensor-head-housing, and this is mostly the axial exit of the cable or the connector from the front face of the sensor-stick-housing opposed to the sensor-head-housing.
However, when depending on the installed solution this front face of the sensor-head-housing is built over the cylinder unit, e.g. through a mounting eyelet required on this side of the piston/cylinder unit, the exit of the cable from the piston/cylinder unit has to be performed towards the side.
The necessary angulation towards the side of the cable exiting the sensor-head-housing alone, again, requires additional axial volume of the piston/cylinder unit.
An initially provided exit direction of the cable, or of the connector, perpendicular to the axial direction is disadvantageous, on the other hand, if in the actual installation situation a further axial track of the cable is required and insufficient space is available on the sides.
Repairs constitute anther problem area.
Due to the described, often high operating pressures in such piston/cylinder units and a rough operating environment, like for instance, strong vibrations as they often occur in equipment, a failure of the piston sensor can occur so that it needs to be replaced completely, or components of it, possibly a part of the processing electronics or of the wave conductor unit of the magnetostrictive sensor.
In this case the piston/cylinder unit had to be opened, until presently, and the whole position sensor with its housing had to be removed from the piston/cylinder unit since, especially due to the above mentioned operating conditions, the processing electronics in the sensor-head-housing of the sensor were generally encased solid.
However, this means that before removing the senso, the operating fluid in the respective piston/cylinder unit has to have ambient pressure, since otherwise large amounts of operating fluid exit into the environment and the connected actuators change positions unintentionally.
On the other hand, after replacing the position sensor, the respective piston/cylinder unit or the whole operating loop to which it is connected may have to be refilled or at least bled, which entails a considerable effort and poses an additional source for failures if performed incorrectly.